The present invention relates to a method of and apparatus for forming thin-walled cups of expanded resin.
There is known a method of forming cups of an expanded material by filling the mold cavity with beads of expandable thermoplastic resin, heating the beads to be expanded and fused into a solid piece and cooling the same. In such method, there are used beads of expandable thermoplastic resin which have been pre-expanded to a predetermined size. After introduced into the mold cavity defined by a pair of female and male members of a mold, the beads are expanded and fused together. The mold cavity should therefore have a space having a sufficient width to allow the beads to pass through and be introduced in the mold cavity. When the mold cavity is filled with the beads, there are produced gaps between the beads due to their spherical shape. In order to assure water-tightness for a cup, it is required to fill up these gaps completely by the expansion and swelling of the beads to make the article compactly formed on the whole, leaving no gaps therein.
In this connection, the mold cavity should be defined so as to have throughout a spatial width more than a predetermined size. This results in the production of expansion-formed articles or cups having a wall thickness more than a predetermined thickness value, since the wall thickness corresponds to the spatial width of the mold cavity.
However, it has become necessary to manufacture thin-walled formed articles dependent on the shapes or applications required. Conventional forming methods could not meet such demand.
For example, when manufacturing slender and deep cylindrical cups to be used for drink, it was difficult to form cups with a wall thickness less than 1 mm by a conventional bead forming method, since the mold cavity requires a spatial thickness of at least 1 mm. That is, in order to produce a good formed article having no defects such as pinholes by the bead forming method, beads should be introduced so that at least two beads are arranged in the thickness direction (U.S. Pat. No. 3,897,899) and the beads as arranged in such state should be expanded and fused together. However, the diameter of each of expandable beads available to be generally used is about 0.6 mm, even for that having a relatively high density of 0.4 g/cm.sup.3. When carrying out expansion forming with the mold cavity of a mold filled with such large beads, two or more beads cannot be arranged in the widthwise direction of the mold cavity having a spatial width of 1 mm or less. Accordingly, when a cup with a wall thickness of 1 mm or less is made according to a conventional bead forming method, the wall inevitably contains pinholes or gaps to provoke a water leak or to lower the water-tightness. Moreover, the mutual fusion of the beads is weak, resulting in reducing strength of the finished article.
On the other hand, the wall thickness of such cups affects a so-called stacking height or the height of cups stacked as fitted in one another. The thinner the wall thickness, the lower the stacking height to improve the housing efficiency in storage and transportation of the cups. Accordingly, it has been long desired to produce cups with a wall thickness less than 1 mm which exceeds the forming limit in the conventional bead forming.
In order to meet such demand, there has been proposed a forming method comprising a step of heating beads to cause the same to be expanded and fused together, thereby to produce a formed article in the form of a cup having a thick wall, and reducing the wall thickness of the formed article by mechanically pushing the formed article or exerting a pneumatic pressure to the formed article from the inside of the mold male member (For example, Japanese Patent Publication No. 8744/1973).
However, when a pneumatic pressure is used in this method, it is difficult to exert such pneumatic pressure uniformly to the formed article in the form of a cup. This results in the occurrence of minute irregularities on the thin-walled portion to disadvantageously lower the uniform thickness accuracy. When the article is to be mechanically pushed, the mold has to be partially moved. This would inevitably complicate the mold structure and the actuating mechanism, thus inducing increase in the mold manufacturing cost and complication of the forming process. Further, when a portion, for example the peripheral side wall, of a formed article of a cup is reduced in thickness by compression, excessive force is exerted to the boundary between the bottom and the peripheral side wall. It is therefore difficult to produce a formed article or cup well balanced from the view of the totality of the formed article. According to this method, further, the same mold is submitted to the heating step after the introduction of beads and the subsequent cooling step. This results in a considerable loss of heating and cooling energies and disadvantageously takes more time for the forming cycle.
In order to meet the demand mentioned earlier, there has been also proposed a method according to which the mold cavity of a mold is defined so as to have a relatively large spatial width at the time of bead introducing process or the heating process, and thereafter the mold is partially moved to compress, for example, the peripheral side wall of a preformed cup, thereby to reduce the wall thickness. (For example, Japanese Patent Laid-Open Publication No. 142668/1975). However, this method also presents a drawback similar to that discussed in connection with the mechanical pushing in the method mentioned earlier.
In expansion forming of not only cups but also general articles, one mold is repeatedly submitted to the bead introducing process, the heating process and the cooling process in succession. This disadvantageously incurs a loss of both heating and cooling energies and takes too much time for the forming cycle.
In order to eliminate such energy loss and to shorten the cycle time, there has been proposed a forming method according to which there are used two molds, i.e. a mold for heating process and a mold for cooling process, and an article formed with the mold for heating process is transferred to the mold for cooling process. (For example, U.S. Pat. No. 4,106,884).
According to this forming method, however, a formed article after heat-forming is transferred under atmospheric pressure, allowing the formed article to expand so that such article undergoes a considerable change in wall thickness. Therefore, this forming method is not suitable to the production of the thin-walled slender and deep cylindrical cups mentioned earlier, but can be merely applied to the production of formed articles for which strict accuracy on the dimensions are not required.
As another forming method using the transfer system, there has been proposed a method according to which, in order to leave no minute gaps between the fusion-bonded beads, a first mold filled with beads is submitted to a heating process to expand the beads, after which the same first mold is submitted to a cooling process such that a resultant formed article won't be deformed even if released from the first mold under atmospheric pressure. The formed article is then released from the first mold and transferred to a second mold under atmospheric pressure. While heated and melted, only the surface of the formed article is compressed in the second mold, and other portions of the formed article are cooled simultaneously with or prior to the compression. (For example, Japanese Patent Laid-Open Publication No. 190335/1985). According to this method, only the surface of the formed article is compressed while heated and melted, and other portions are merely cooled. This would produce a hard resin layer on the surface, but the entire formed article or cup is difficult to compress whereby the wall thickness cannot be readily reduced. Likewise in normal forming, the first mold is submitted to both the heating process and the cooling process. Therefore, this method does not contribute to elimination of the energy loss and reduction in the forming cycle time.
In order to eliminate a loss of energy and shorten the forming cycle time, there have been proposed two methods. According to one method, a formed article is transferred from a heating mold to a cooling mold in a controlled atmosphere, instead of under atmospheric pressure, in order to restrain deformation due to the expanding force of the formed article (For example, Japanese Patent Laid-Open Publication No. 98149/1981). According to the other method, a preformed article formed to a predetermined configuration is transferred from a heating mold to a cooling mold in a state allowing the preformed article to freely expand (For example, U.S. Pat. No. 4,260,571).
However, these methods merely deal with the transfer of a formed article from the heating step to the cooling step either in a controlled atmosphere or in such atmosphere as to allow the formed article to be freely expanded, but do not include a step of compressing the formed article or cup to reduce the wall thickness thereof.
Instead of such bead forming, there has been also used a so-called sheet forming method in order to produce thin-walled formed articles. According to the sheet forming, after heated and softened, an expanded sheet preliminarily extruded is raised and deformed along the mold into a predetermined configuration. According to such sheet forming, however, it is difficult to form deep cups having a greatly raised configuration such as cups for drink, since the sheet cannot be sufficiently oriented. Further, a finished article is made from a single sheet. It is therefore difficult to form a finished article partially thickened for the purpose of reinforcement. Moreover, restrictions are imposed on the structure of a sheet forming mold, thus bringing limitations to the configuration of a formed article to be obtained.
It has been therefore long desired to manufacture thin-walled formed articles in a short period of time with a loss of energy minimized, according to the bead forming method having various advantages as compared to the sheet forming method.